Select the true statement.

Options

A. In fishes kidney play a major role in ammonia excretion
B. Ammonia is 100,000 times less toxic than urea
C. Sharks retain a large amount of urea in the blood as a major osmolyte to balance the osmolarity of the body fluids
D. Most terrestrial reptile excrete ammonia

Answer

To determine which statement is true, we will analyze each statement one by one:

1. **Statement 1**: "Kidney plays a major role in ammonia excretion."
- **Analysis**: This statement is incorrect. In fish, ammonia is primarily excreted through the gills, not the kidneys. Fish have a different mechanism for excretion compared to terrestrial animals, and they do not rely heavily on kidneys for ammonia excretion.

2. **Statement 2**: "Ammonia is 1 lakh times less toxic than urea."
- **Analysis**: This statement is also incorrect. Ammonia is actually more toxic than urea. Urea is a less toxic form of nitrogen waste that is excreted by many terrestrial animals, while ammonia is highly toxic and requires a lot of water to be safely excreted.

3. **Statement 3**: "Sharks retain a large amount of urea in blood as a major osmolite to balance osmolarity of the body fluid."
- **Analysis**: This statement is true. Sharks and some other marine animals retain urea in their blood to help maintain osmotic balance with their surrounding environment. This adaptation allows them to survive in a marine environment where they would otherwise lose water.

4. **Statement 4**: "Most terrestrial reptiles excrete ammonia."
- **Analysis**: This statement is incorrect. Most terrestrial reptiles are uricotelic, meaning they excrete nitrogenous waste primarily in the form of uric acid, which is less toxic and conserves water.

**Conclusion**: The only true statement is **Statement 3**.